Electrodynamic loudspeaker



July 13, 1948. F. MASSA 2,445,276

ELECTRODYNAMIC LOUDSPEAKER Filed May 4, 1945 2 Sheets-Sheet 1 EM 5 IFig. 3

VOLUME OF CONDUCTOR cg CM.

Fig-5 3M 77W "4. I0 I 20 30 v CONE DIAMETER INCHES ISnnentor July 13,1948. F. MASSA 2,445,276

ELECTRODYIiAMIC LOUDSPEAKER Filed May 4, 1945 2 Sheets-Sheet 2 PER CENTFFlCIENCY 0 v I0 I00 VOICE COIL MASS IN GRAMS 5 3nvento4:

Patented July 13, 1948 UNITED STAT ES PATENT OFFICE 2,445,276ELECTRODYNAMIC Ldfiifsr' ilafifiit Frank Massa, Cleveland fieighta dliioAppneafion May 4,1945, serial N6; 591932 8 Claims. (Cl. wa -115 .5

My invention is concerned with loud speakers; and, more; particularly;with the direct-radiator elctro dynaniic' type of loud speaker. In theelectro-dynamic speaker, the driving force is created in a coilof wiresupplied with current and residing in a magnetic field. The coil of wireis" usually connected to a vibr'atable cone such that vibrations of thedone are produced corresponding to the variations in the voice coilcurrents.

It has been universally assumed by loud speaker design engineers" that adirect-radiator speaker is inherently a low-efficiency unit because ofthe inherently low radiation resistance which is presented to avibrating cone especially at the lower frequencies. This general feelingthat it? iiapasnbre to obt'ain hig'h efi icincie s' in a dirtr'a-di'atoi dynafi'iic speaker has led t'o the widespread use of hornsin installations requiring high efiiciencies in the conversion ofelectrical energy to eases-ti energy. Contrary to this accepted generalbelief, I have found it pos silSl' to (resign direct-radiator speakersthat have very high efficiencies, comparable to the eiiicienciesrealized in horn type loud speakers. By employing theteachin' g's ofni'yinvention as set forth in this specification, it will be possible t6produce low-frequency direct radiatdr loud speakers which are rio't onlyvery much more emcient than the eflicinoi'e's' now generally ob"- tainedfrom this type of speaker, but it will also possible to obtainthis-improved performance in a unit having a much smaller cubic volumetheir is now needed in' the equivalent horn type ciencies ofthe order of5% and, in many inv been overlookedin this line of reasoningisthatlow-uual-ity transient reproduction will generally result in alow-efiiciency speaker. With im'- proved quality of radio broadcasting,the defects resulting from afllowjelificiency speaker will cause 2' ttire-101m speaker sys'tehj can ne er exceeq loo- 4, efn eaey if; forexample, the general em: cieii 035 are speaker" is the peak eifeed" 3did. at height. For ffielncy Speaker, however, are same peak would be 13db". iriliei'glit r a another object of my invention is to'obtain awhich occupies much less" space than would be" requiredby' eZ horn type"l'oud speaker havin'g comparable emci'en'c'y;

A'sti ll fufthei object or my invention is to de: fine the mass ofconductor material which must be employed in the construction of thevoice coil for' various diameters of diaphragm's" in order that higliefil'iencies may be realized at thelower audio frequencies.

Anotlier object of my invention is to employ an aluminumvoice coil of aspecified ubie' volume iii-relation to'the diameter of the vibrat;

' in'g diaphragm iii order that high citricien'cies A still furtherobjiiti of my" illveritioniis to provide" avoice coir or uniqueconstruction sun that rghest efiicien'c'i'es may be obtained in a directraidiator dynamic speaker having a? fixed arrest- 5 dimension: r

Another obj c'if' invention is to; define relationship b we'e'ii fluxdeiisit size of diameagre and weight (if voice 0611 that will yield highef ciencies for dynamic speaker over a wide wry-frequency range ofreproduction.

A5 mailer (street or m' y menialw d'efiii'e" magma between are eerie;vomme hat tee ployed ve an-tuna maven-or my'new' tic of my invention areset forth with particularity in the appended claims. The invention,itself, however, both as to its organization and method of operation, aswell as advantages thereof, will best be understood from the followingdescription of several embodiments thereof, when read in connection withthe accompanying drawings, in which Fig. 1 is a simple diagrammatic Viewof a dynamic speaker mounted in a flat bafile and with reference towhich the analysis for the conditions necessary to obtain highefiiciences will be made. i

Fig. 2 is a diagrammatic view showing a dynamic speaker mounted insidean'enclosed volume such that only one side of the vibrating diaphragmmay transmit energy to the atmosphere.

Fig. 3 is a circuit illustrating the impediance presented to anelectrical circuit by the loud speaker systems shown in Figs. 1 and 2.

Fig. 4 is a set of curves which indicate the efiiciencies of adirect-radiator dynamic speaker having various sizes of vibratingdiaphragms, as a function of the voice coil mass employed in theconstruction of the diaphragms. separate set of curves is shown for thecase of aluminum and copper voice coils.

Fig. 5 is a graph showing the optimum volume of aluminum conductor to beemployed as a function of cone diameter in order to realize maximumeiiiciency in the low-frequency region. Another curve is shownindicating the minimum volume of aluminum conductor that is required asa function of cone diameter if a net gain in efficiency is to berealized over the use of copper in the same voice coil structure.

Fig. 6 shows a novel voice coil construction for permitting a maximumamount of conductor to be built economically into a coil of specifiedlinear dimensions.

Fig. 7 illustrates the method of folding the voice coil ribbon to permitthe construction of the voice coil terminals indicated in Fig. 6.

Referring more particularly to Fig. 1, the reference character I refersto a diaphragm which is flexibly mounted to a supporting rim 2 which, inturn, is'fastened to a flatbafile 3, as is conventionally done in loudspeaker installations. A magnetic structure 4 furnishes flux in acircular air gap in which is mounted the voice coil 5. The voice coil 5is rigidly attached to the diaphragm i by any of the well-known meansemployed in the art. The metal framework which ties the magneticstructure Q to the supporting rim 2 is not shown since this figure isdiagrammatic and such physical structures are well-known in the art andform no part of this invention.

In Fig. 2, is shown the same speaker structure described in connectionwith Fig. 1 except that the speaker is mounted inside a totally enclosedrigid box 6 which provides a net volume of V cubic inches of air behindthe diaphragm i.

In order to show the specific requirements that must be met for therealization of high efiiciency in the low-frequency region, referencewill be made to the simplified circuit shown in Fig. 3 in which RE isthe electrical resistance in ohms of the voice coil conductor 5 and ZEMis the electrical impedance reflected into the electrical circuit due tothe motion of the cone. The magnitude of this reflected mechanicalimpedance will be different for the operating conditions shown in Figs.1 and 2, and these differences will be pointed out later. In the circuitof Fig. 3, I represents the current in amperes flowing through the voicecoil under a specified condition of operation. The assumptions whichhave been made in connection with setting up the electrical circuitshown in Fig. 3 is that the stiffness of the cone suspension in Figs. 1and 2 is low enough to permit the diaphragm l to operate as a pure massin the frequency region concerned, and in the case of Fig. 2, the volumeV is large enough so as not to restrain materially the vibration of thecone I over the frequency range of interest.

The magnitude of ZEM in Fig, 3 is expression given by the where B=airgap flux density in gauss When a current of I amperes flows through thevoice coil, the electrical power input to the loud speaker system isgiven by PE=I (Rr+real part of Zen) where M0=MV+MC+MA For the current Iamperes flowing through the voice coil, a driving force will begenerated in the coil equal to BL] FM1'0 dynes This force will result ina velocity of the diaphragm equal to F V m cm./sec. l)

Substituting Equation 3 into Equation 4, it can be shown that theabsolute value of the diaphragm velocity squared will be B L I 10- 1mCD1.2/S8C.2

The acoustic power output from the diaphragm is given by PA=IVI RMergs/sec. (6)

By substituting Equation 5 into Equation 6 and multiplying by 16' toconvert ergs/sec. into watts, the acoustic power output becomes 9R2+w2M02 watts (7) Since the efficiency is the ratio of the acousticpower output to the electrical power input, the

efficiency is equal to the ratio of Equation '7 over Equation 2, whichbecomes Efficiency EU M zM 2) R ,B L X10 In order to make Equation 8more generally applicable to the analysis of a loud speaker in terms ofthe basic constants of the voice coil conductor,1 may substitute for Bein Equation 8 its equivalent, namely,

' PDL'.

where =reslstivity of voice coil conductor in ohms/cm. D=density ofvoice coil conductor in -gms./cc.

My /(M +M grams In order to learn how the various design parameters of adirect-radiator loud speaker affect the efficiency of operation, I havesolved Equation 10 for four different effective cone diameters, namely,6", 10", 18", and 30. The loud speaker was assumed mounted in a largebaflle, as shown in Fig. 1. Several voice coil sizes were employed inthe computations and both aluminum and copper conductors wereinvestigated. The results of this analysis are indicated in-the graphsshown in Fig. 4. The curves plotted in Fig. 4 show the per centefliciencies realized by the various cone diameters as marked on .eachcurve .as .a function of the voice coil mass shown along the abscissa.The upper set of curves are for .an aluminum voice coil and the lowerset of curves represent the same cones driven by copper voice coils. Forall of the data plotted :in Fig. 4, the air gap .fiux density is 10,000gauss. At higher flux densities, the efficiencies are higher and atlower flux densities, the eiliciencies are lower than indicated.

I In 'order that the computed efliciencies would have practicalsignificance, I used typical cone masses as follows: 6" diameter-:2grams; 10" diameter=l0 grams; 18" -diameter==40 grams; and 30"diameter=150 grams. The efficien y curves shown in 4 apply approximatelyfor the entire lower audio-frequency range in which the wavelength isgreater than approximately one-third the diameter of the vibratingdiaphragm.

Some rather basic and here-to-fore unknown information is brought tolight by an examination of Fig. 4. Some of these facts include:

1. It is possible to produce direct-radiator dynamic speakers havingefficiencies in the neighborhood of 50% :by employing larger rconediame'ters and much heavier voice coils than have been generally assumedto be adequate.

2. By simply increasing "the diameter of the cone, the efficiency of :aparticular :loud speaker will actually decrease unless the voice coilweight is very materially iincreased. This decreased emci-enicy from alarge diaphragm has been experimentally :observed and has led to thegeneral use of cones having .eifective vibrating diameters less than 12inches because it was notrrecognized that the decreased efiiciencyproduced by the larger cone wasthe result of-a lack --'of understandingof the necessary voice :coil requirements :as ;set forth in thisspecification. .In -fact, it .has become conventional practice to employtwo small speakers in a radio set to improve the low irequency performance without recognizing that one large speaker may be designed tobe more efflcient. For the fewexceptional cases where larger cones havebeen employed, the low'emciency has been toler ated for the sake ofobtaining larger power han: dling capacity (due to less requiredamplitude of the larger cone for a required output power). The voicecoil weights have been far below the values I found necessary forhigh-.e-fiiciency operation.

3. The absolute efilciency or a direot radiator loud speaker mayincrease with increasing cone diameters provided the voice coil massincreases approximately as the cube of the increase in the diameter ofthe cone.

4. The empirical relationship that can be deduced for the conditionsshown in Fig. 4 is that r in a direct-radiator loud speaker mounted inan infinite bafiie, maximum low-frequency efficiency is realized whenthe mass of the voice coil condoctor in grams is equal to approximatelyD /50, where D is the .efiective diameter in inches of the vibratingdiaphragm.

5. Theregion of maximum eificiencies for the various diaphragms issufliciently broad so that the optimum voice coil mass is, fortunately,not too critical a quantity. However, if the voice coil mass is reducedbelow approximately one-fifth of the optimum value, the efliciencybegins to fall or! fairly rapidly. This "means that the desirable regionwithin which the voice coil mass should lie in order to profit from thehigh efliciency possibilities is approximately D /250 to D /50 grams,where Dis the effective diameter of the voice coil in inches. I

6. It is apparent from the information plotted in :Fig. .4 that "thegeneral practice of employing effective cone diameters up to about 12"for low.- frequency reproduction causes considerable sacrifice in theefdciencies that would be realized if larger diameters were used.

7. Aluminum used as the voice coil conductor is much superior .to :acopper coil of the same mass; however, the volume of an aluminum coilhaving the same mass as a copper coil would be over three times greaterthan the volume required for the copper coil, which means that a muchmore costly magnetic circuit would be required.

A further examination of the data shown in,

Fig. *1 indicates that it is possible to improve the e'fliciency, undercertain conditions, if aluminum is substituted for copper withoutincreasing the volume of the air gap. In order to show under whatconditions this situation is possible, the .efl'iciency data shown inFig. 4 were converted from voice coil mass .as a parameter to theequivalent voice coil volume, and from this new set of data, the solidcurve in .Fig. '5 was derived. This curve shows the volume of conductoras a function of cone "diameter at which the efficiency is the samewhether the conductor is aluminum or copper. For voice coil volumeslarger than the values indicated by the solid line, an absoluteimprovement in .eiiicien'cy will result :if aluminum is employed in theconstruction. The dotted. curve in Fig, '5 represents the cubic volumeof aluminum neces sary to yield the maximum efliciency for various conediameters and it is merely the plot of the highest points of theefficiency curves shown in the upper half of :Fig. "4. The region"between the solid and dotted lines, therefore, represents a range .of'voice .coi'l volumes which will give improved .emc'iencies withaluminum as the conductor rrafiher ithan copper. The empiricalrelationship which may be deduced from these data is that in order toprofit from the use of aluminum in the construction of a voice coil, thevolume of the voice coil conductor must lie within the approximate rangeD /850 cc. to D 135 00., where D is the effective diameter of thediaphragm in inches.

The empirical relationshipsthat I have shown specifying the optimumdesign conditions for voice coils and diaphragm have employed typicalcone weights which are representative of the various sizes mentioned.Small deviations from these chosen weights will produce only minorchanges in the values shown in Figs. 4 and 5 and, therefore,'the rangesof empirical values which I have specified for the voice coil designwill be generally correct for engineering purposes.

All the efiiciency data described above have been determined for adirect-radiator loud speaker mounted in an infinite bailie, as shown inFig. 1. If the loud speaker is mounted so that only one side radiates,as shown in Fig. 2, the general set of efficiency curves will be shiftedslightly to the left as compared with the curves plotted in Fig. 4. Thisshift results from the somewhat reduced effective mass and radiationresistance of the vibrating system under this condition of installation.For this condition, the optimum range within which the voice coil massmust lie in order to obtain high eficiency is approximately .7 thevalues specified under items 4 and 5 in column 6 and also about .7 thevalues given at the end of the above paragraph. This means that theoptimum range of the voice coil mass in grams for the condition of Fig.2 lies between D /350 and D /70, where D is the effective diameter ofthe vibrating diaphragm in inches. The range of voice coil volume withinwhich improved efficiency will result by the use of aluminum instead ofcopper for the condition of Fig. 2 becomes D /1200 to D /190 cubiccentimeters, where D is the effective diaphragm diameter in inches.

In addition to permitting a somewhat reduced voice coil size for thesame high efilciency operation, the totally enclosed type of mounting ofFig. 2 will permit a much smaller structure than the rather large bafilewhich is required for the arrangement of Fig. 1. For the typical conesemployed in the determination of the efllciency data described above, Ihave found that the approximate volume of the enclosure, in order thatresonance of the cone and volume shall lie below 80 cycles, must begreater than 200 D cubic inches, where D is the diameter of the cone ininches.

Fig. 6 shows a diagrammatic cross-sectional view through a cone andvoice coil assembly indicating a new method for constructing the voicecoil in order that practically the entire volume of the coil is occupiedby active conductor. The voice coil 5 is wound from a continuous, thin,flat ribbon l which is also shown in the plan view of Fig. 7 indicatinghow the ribbon is folded in order to create a tab which forms the voicecoil terminal 8 in Fig. 6. Several layers of the ribbon are wound on acoil form 9 with an insulating film interposed between each layer, andthe entire voice coil structure forms a relatively rigid cylinder whichis intimately attached to the voice coil form 9. The inside tab of theribbon I is cemented along the voice coil form, as shown in Fig. 6, andthe outside tab is folded back and cemented over an insulating strip 10and then cemented along the voice coil form to establish the secondvoice coil terminal 8, as indicated. In order to gain maximum advantagefrom a given air gap,

it is generally desirable to make the width of the ribbon employed asthe conductor in the voice coil construction of Fig. 6 somewhat greaterthan the axial length of the air gap in the magnetic structure. In thisway, the voice coil will make use of the leakage flux as well as thestraight air gap flux in its operation.

Although the quantitative information shown in this specification wasderived for specific sets of vibrating diaphragms, it is obvious thatminor changes in the results shown would be produced if a somewhatheavier or lighter cone were employed than the particular values chosen.These minor variations, however, will not materially affect the resultsdisclosed. Other obvious changes in some of the details which have beenmentioned may be made by those skilled in the art without departingsubstantially from the teachings of this invention, and I, therefore,desire that my invention shall not be limited except insofar as is madenecessary by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. In combination in a loud speaker, a magnetic structure having an airgap, a coil of electrically conducting material located in said air gap,a direct radiating diaphragm rigidly attached to said coil and theassembly flexibly suspended so that it may freely vibrate when driven byforces generated in said coil, the mass in grams of said coil lying inthe range between D /250 and D /50 where D is the effective diameter ininches of the vibrating area of said diaphragm.

2. The invention set forth in claim 1 characterized in that theeffective diameter of the diaphragm is greater than 12 inches.

3. In combination in a loud speaker, a magnetic structure having an airgap, a coil of aluminum conductor located in said air gap, a diaphragmrigidly attached to said coil and the assembly flexibly suspended sothat it may freely vibrate when driven by forces generated in said coil,the volume of said conductor lying in the range D /850 cubic centimetersand D /l35 cubic centimeters, where D is the ellective diameter ininches of the vibrating area of said diaphragm.

4. The invention set forth in claim 3 characterized in that theeiiective diameter of the diaphragm is greater than 12 inches.

5. In combination in a loud speaker, a magnetic structure having an airgap, a coil of electrically conducting material located in said air gap,a diaphragm rigidly attached to said coil and the assembly flexiblysuspended so that it may freely vibrate when driven by forces generatedin said coil, the mass of said coil lying in the range between D /350and D /70, where D is the eiiective diameter in inches of the vibratingarea of said diaphragm, and an enclosing structure surrounding one sideof said diaphragm such that sound radiation takes place from only theunenclosed side of said diaphragm.

6. The invention described in claim 5 further characterized in that thediaphragm has an effective diameter greater than 12 inches and thevolume contained within said enclosing structure is greater than 20ODcubic inches, where D is the effective diameter in inches of thevibrating area of said diaphragm.

7. In combination in a loud speaker, a magnetic structure having an airgap, a coil of aluminum conductor located in said air gap, a diaphragmrigidly attached to said coil and the assembly flexibly suspended sothat it may freely vibrate when driven by forces generated in said coil,the volume of said conductor lying in the range D /1200 cubiccentimeters and D 190 cubic centimeters, where D is the effectivediameter in inches of the vibrating area of said diaphragm, and anenclosing structure surrounding one side of said diaphragm such thatsound radiation takes place from only the unenclosed side of saiddiaphragm.

8. The invention described in claim 7 further characterized in that thediaphragm has an efl'ective diameter greater than 12 inches .and thevolume contained within enclosing structure is greater than 200D cubicinches, where D is the effective diameter in inches of the vibratin areaof said diaphragm.

FRANK MASSA.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,631,646 Rice June 7, 19271,690,840 Round Nov. 6, 1928 1,853,721 Bryson Apr. 12, 1932 1,861,222Minton May 31, 1932 1,953,542 Pridham Apr. 3, 1934 2,039,854 Stevens May5, 1936 2,039,856 Toher May 5, 1936

